Download Electricity is a form of energy resulting from the existence of charged

Basic Electricity
ME 120 Lecture Notes
Portland State University
Mechanical and Materials Engineering
Learning Objectives
Successful completion of this module will enable
students to
• Link the basic model of an atom to the flow of
electricity
• Apply the definitions of Amp, Volt, Coulomb,
Joule, Watt to unit conversions and basic
problems involving current and voltage
• Apply Ohm’s Law to simple DC circuits
Definition
Electricity is a form of energy resulting
from the existence of charged particles
(such as electrons or protons), either
statically as an accumulation of
charge or dynamically as a current.
Concise Oxford English Dictionary, revised 10th edition
Definition
Electricity is a form of energy resulting
from the existence of charged particles
(such as electrons or protons), either
statically as an accumulation of
charge or dynamically as a current.
Concise Oxford English Dictionary, revised 10th edition
Definition
Conductor:
A conductor is a material that
readily allows the flow of electricity. A
good conductor has a high numerical
value of a conductivity, and a low
numerical value of resistance.
Definition
Conductivity:
All materials have a measurable
property called electrical conductivity
that indicates the ability of the
material to either allow or impede the
flow of electrons. Materials that easily
conduct electricity have a high
conductivity.
Definition
Insulator:
An insulator is a material that tends
to impede the flow of electricity. An
insulator resists the flow of electricity.
A good insulator has a low numerical
value of conductivity and high
numerical value of resistance.
Definition
Resistance:
Resistance is a quantitative indicator
of an objects ability to act as an
insulator. A high resistance wire will
impede the flow of electrons. A low
resistance wire will allow a greater
flow of electrons.
Definition
Semiconductor:
A semiconductor is a material with
conductivity that can change between
that of a conductor and insulator.
The conductivity of a semiconductor
can be changed by exposing it to a
stimulus such as an electrical field,
light, mechanical pressure, or heat.
Simplified Functional Differences
Semiconductors can be used
in devices that act like a
switch.
Elements
• Pure substances are made of elements.
• An element consists of atoms
• Atoms have a nucleus consisting of protons
and neutrons
• Electrons move in shells around
the nucleus
Chemical properties of conductors
To understand the mechanisms of electron flow,
we will take a brief look at the chemical
properties of matter.
Elements
• Number of protons determines the element
• Number of electrons varies
– State of electrical charge
– Is the element in a chemical bond?
• Number of neutrons varies with isotope
Periodic Table: Copper
Bohr Model of the atom (Cu)
Electrical current in a trivial circuit
Electrical current: atomic model
Electrical Current: electron flow
Electrical Current: electron flow
Electrical Current: current convention
How many electrons?
How many electrons?
Electrical current: potential
Electrical current: electron flow
Electrical current: convention
Definition: Charge
Elementary charge
1 electron = 1.602  10–19 coulomb
Coulomb
1 coulomb= 6.24  1018 electrons
Definition: Current
Electrical current is the flow of the electrons.
C
1A = 1
s
18
1 C = 6.24 ´10 electrons
Definition: Voltage
Voltage is the energy potential that would cause
electrical charge to move from a region with
relatively more electrons to a region with
relatively fewer electrons.
J
1V = 1
coulomb
Voltage and electrical work
If the voltage between A and B is one
volt, then one Joule of work is done
when 6.28 × 1018 electrons move from
A to B.
Voltage is a Potential
A high voltage between two regions does not
guarantee that a lot of electrons will flow.
The electrical resistance between the two
regions determines the rate of electron flow for
a given voltage.
Ohm’s Law
Ohm’s law describes the relationship between
voltage, current and resistance, when current
flows through a wire or resistor.
V
R
I
V = IR
Ohm’s Law
Ohm’s Law
Ohm’s Law
Ohm’s Law
Summary (1)
Materials can be characterized by their ability to
conduct electricity
Conductors
Readily allow the flow of current
Resistors
Impede the flow of current
Semiconductors Act like conductors or resistors
depending on external stimulus
Elements with lonely valence electrons are good
conductors
Summary (2)
Electrical current is caused by the flow of
electrons
By convention, we say that current flows from
regions of positive charge to regions of negative
charge
Summary (3)
The unit of electrical charge is coulomb (C).
The unit of electrical current is amp (A)
C
1A = 1
s
18
1 C = 6.24 ´10 electrons
Summary (4)
Voltage is the energy potential that tends to
cause electrical charge to move.
J
1V = 1
coulomb
Summary (5)
Ohm’s law relates voltage, current and
resistance for the flow of current through a
conductor.
V
R
I
V = IR